Conventionally, a control system for an internal combustion engine, which controls the amount of intake air drawn into cylinders of the engine via a variable intake mechanism, has been proposed in Patent Literature 1. This control system is comprised of an air flow sensor that detects the flow rate of air flowing through an intake passage of the engine, a crank angle sensor that detects a state of rotation of a crankshaft, an accelerator pedal opening sensor that detects an opening of an accelerator pedal (hereinafter referred to as “the accelerator pedal opening”), and a controller to which are input detection signals from these sensors. The controller calculates an engine speed based on the detection signal from the crank angle sensor, and the amount of intake air (intake air amount) based on the detection signal from the air flow sensor. Further, the engine is provided with a throttle valve mechanism and a variable valve lift mechanism as variable intake mechanisms. The throttle valve mechanism changes the flow rate of air flowing through the intake passage as desired, and the variable valve lift mechanism changes the lift of each intake valve (hereinafter referred to as “the valve lift”) as desired.
As will be described hereinafter, in the control system, the intake air amount is controlled by the controller. First, it is determined based on the engine speed, the accelerator pedal opening, and the intake air amount, in what load region the engine is operating. Then, when it is determined that the engine is in a low-engine speed and low-load region including an idling region, the valve lift is controlled to a predetermined low lift by the variable valve lift mechanism, and the opening of the throttle valve is controlled to a value corresponding to the engine speed and the accelerator pedal opening by the throttle valve mechanism. On the other hand, when it is determined that the engine is between a medium-engine speed and medium-load region and a high-engine speed and high-load region, the throttle valve is controlled to a fully-open state, and the valve lift is controlled to a value corresponding to the engine speed and the accelerator pedal opening.
[Patent Literature 1] Japanese Laid-Open Patent Publication (Kokai) No. 2003-254100
In the control system proposed in Patent Literature 1, it is sometimes impossible to properly calculate the intake air amount, due to a low resolution of the air flow sensor. For example, some type of internal combustion engine has an intake passage whose diameter is set to a large value (i.e. which is set to a large diameter) so as to reduce flow resistance within the intake passage to enhance the charging efficiency of intake air into cylinders. When the above control system is applied to the engine configured as above, the flow velocity of intake air assumes a very low value when the engine is in a low-engine speed and low-load region, and hence the control system cannot properly calculate the intake air amount due to the low resolution of the air flow sensor, which degrades the accuracy of the intake air amount control. As a result, when the air-fuel ratio of a mixture in a combustion chamber is controlled based on the intake air amount thus calculated, there is a fear that the resulting degraded control accuracy lowers fuel economy and increases exhaust emissions.
On the other hand, in the ignition timing control of the engine, a method is conventionally employed which uses an engine speed and an intake air amount as load parameters indicative of load on the engine, and ignition timing maps having map values of ignition timing set in advance in association with the load parameters. In the above-described engine with the intake air passage having a large diameter as well, it is envisaged that ignition timing is controlled by the above method. However, as described above, in the control system proposed in Patent Literature 1, the intake air amount cannot be properly calculated in the low-load region of the engine due to the low resolution of the air flow sensor. This degrades the accuracy of the ignition timing control.
A control system for an internal combustion engine, which is capable of solving the above problems of the conventional control system, has been proposed in Japanese Patent Application No. 2004-133677 by the present assignee. This control system is comprised of an air flow sensor that detects the flow rate of air flowing through an intake passage of the engine, a pivot angle sensor that detects the valve lift, a cam angle sensor that detects the phase of a camshaft for actuating an intake valve to open and close the same with respect to a crankshaft (hereinafter referred to as “the cam phase”), and a crank angle sensor. Further, the engine includes the intake passage having a large diameter, a variable valve lift mechanism, and a variable cam phase mechanism as variable intake mechanisms. In the engine, the valve lift and the cam phase are changed by the variable valve lift mechanism and the variable cam phase mechanism as desired, respectively, whereby the intake air amount is changed as desired.
In the above control system, in the low-load region of the engine, as the intake air amount, a first estimated intake air amount is calculated according to the valve lift and the cam phase, and in the high-load region of the engine, a second estimated intake air amount is calculated according to the flow rate of air. In a load region between the low-load region and the high-load region of the engine, a weighted average value of the first and second estimated intake air amounts is calculated. Furthermore, air-fuel ratio control and ignition timing control are carried out using the thus calculated intake air amount. As a result, by employing the first estimated intake air amount higher in reliability in the low-load region where the reliability of the second estimated intake air amount is lower than that of the first estimated intake air amount due to the large diameter of the intake system of the engine, and employing the second estimated intake air amount higher in reliability in the high-load region in which occurs a state opposite to the above state in the low-load region, it is possible to further enhance the accuracy of the air-fuel ratio control and the ignition timing control compared with the control system proposed in Patent Literature 1.
According to the control system, however, when detection signals from the pivot angle sensor, the cam angle sensor, and the crank angle sensor drift due to changes in temperature, for example, or when the dynamic characteristics of the two variable mechanisms (i.e. the relationship between the valve lift and the cam phase with respect to control inputs) are changed by wear, contamination, play caused by aging, etc., occurring in component parts of the variable valve lift mechanism and the variable cam phase mechanism, the reliability of the results of detection by the sensors lowers. This can hinder the first estimated intake air amount from properly representing an actual intake air amount and cause the same to deviate from the actual intake air amount. When such a state occurs, if the engine is in the low-load region where the first estimated intake air amount is used as the intake air amount, it is impossible to properly calculate a fuel amount and ignition timing, which can degrade the accuracy of the air-fuel ratio control and the ignition timing control. More specifically, there is a possibility that the air-fuel ratio and the ignition timing assume improper values, which results in unstable combustion and degraded combustion efficiency.
The present invention has been made to provide a solution to the above-described problems, and an object thereof is to provide a control system for an internal combustion engine, which is capable of properly performing air-fuel ratio control and ignition timing control according to an actual intake air amount even when reliability of results of detection of an operating condition of a variable intake mechanism is low.